What is the update_mapping message in logging-es logs

Issue

• We see a number of logs in the logging-es pod logs with a message, update_mapping

[2019-06-21 20:34:23,871][INFO ][cluster.metadata         ] [logging-es-data-master-3a6qmm33] [project.example.81e5b90c-763a-11e8-9102-0050569c31da.2019.06.21] update_mapping [com.redhat.viaq.common]

• What is the cause of this metadata field change in Elasticsearch?
• We see this constantly logged for one project but not for others
• We enabled MERGE_JSON_LOG in our fluentd and now ES is updating its mapping frequently